Losing weight and gaining weight (muscle) use the same principles but in the opposite direction. For example, you need to limit your carb intake to lose weight, and increase it to gain weight. The same goes with exercise — you need to increase your time in the gym lifting weights and doing cardio to burn more calories and drop the pounds. To gain size, you need to spend less time in the gym so the calories are put towards gaining muscle and allowing your muscles to heal and become bigger and stronger. With that said, I do agree with your approach of training a body part 1 time per week. My only suggestion would be to change the split (i.e. chest and biceps could easily become chest and triceps) at least every 6 weeks.
Inspire to Aspire,